[RFC PATCH 15/15] nvme-pci: Allow mmaping the CMB in userspace

[Logan Gunthorpe]

On 2020-11-06 10:39 a.m., Jason Gunthorpe wrote:
> On Fri, Nov 06, 2020 at 10:00:36AM -0700, Logan Gunthorpe wrote:
>> Allow userspace to obtain CMB memory by mmaping the controller's
>> char device. The mmap call allocates and returns a hunk of CMB memory,
>> (the offset is ignored) so userspace does not have control over the
>> address within the CMB.
>>
>> A VMA allocated in this way will only be usable by drivers that set
>> FOLL_PCI_P2PDMA when calling GUP. And inter-device support will be
>> checked the first time the pages are mapped for DMA.
>>
>> Currently this is only supported by O_DIRECT to an PCI NVMe device
>> or through the NVMe passthrough IOCTL.
>>
>> Signed-off-by: Logan Gunthorpe <logang at deltatee.com>
>>  drivers/nvme/host/core.c | 11 +++++++++++
>>  drivers/nvme/host/nvme.h |  1 +
>>  drivers/nvme/host/pci.c  |  9 +++++++++
>>  3 files changed, 21 insertions(+)
>>
>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>> index f14316c9b34a..fc642aba671d 100644
>> +++ b/drivers/nvme/host/core.c
>> @@ -3240,12 +3240,23 @@ static long nvme_dev_ioctl(struct file *file, unsigned int cmd,
>>  	}
>>  }
>>  
>> +static int nvme_dev_mmap(struct file *file, struct vm_area_struct *vma)
>> +{
>> +	struct nvme_ctrl *ctrl = file->private_data;
>> +
>> +	if (!ctrl->ops->mmap_cmb)
>> +		return -ENODEV;
>> +
>> +	return ctrl->ops->mmap_cmb(ctrl, vma);
>> +}
> 
> This needs to ensure that the VMA created is destroyed before the
> driver is unprobed - ie the struct pages backing the BAR memory is
> destroyed.
> 
> I don't see anything that synchronizes this in the nvme_dev_release()?

Yup, looks like something that needs to be fixed. Though I'd probably do
it in the pci_p2pdma helper code instead.

Logan